Who invented the cryptarithms?

The earliest known cryptarithms were found in China. So the Chinese have been given the credit for devising this kind of math puzzles.

During the Middle Ages, appeared in India a genre of cryptarithms nowadays known as "mathematical restorations" or "skeletons".

The great work of boosting and popularizing modern cryptarithmetic was done by the editors and readers of Sphinx, a journal of recreational mathematics, which circulated in Belgium from 1931 to 1939.